The present invention relates to a process, a system and equipment for the towing of underwater pipelines.
The use of methods and relative equipment for the formation of sections of underwater pipelines by the towing of prefabricated “strings” of pipe from the production site to the final destination, is known in the oil industry. Among the various possible methods, the so-called “off-bottom tow” is considered as being the most widely-used. The pipeline is towed by conveying it slightly above the seabed. Specific floats are arranged along the pipeline to guarantee a slightly positive buoyancy whereas segments of chain are arranged in correspondence with the floats, as described, for example, in U.S. Pat. No. 4,011,729. The chain allows the buoyancy to be compensated in relation to the length of suspended section and the pipe therefore has a neutral layout. Thanks to the length of chain, uncertainties with respect to the weight in water of the pipe can be compensated and consequently the buoyancy of the floats. As the pipe does not drag along the seabed, there is no resistance to its advancing. This is due only to the section of chain resting on the seabed (of a length which cannot be accurately defined a priori). Although this method is used, it has various drawbacks among which the fact that, in the presence of transversal currents, the pipe can be shifted off course due to the limited transversal resistance offered by the chains. This problem has been faced, for example in U.S. Pat. No. 4,107,933 and U.S. Pat. No. 4,138,853, by proposing the use of various lay-outs of the chains and/or mechanical drifts to increase the transversal rigidity. In other cases however, it is the very presence of the chains which is not acceptable (excessive uncertainty as to the effective weight of the pipeline, excessive friction of the chains on the seabed, extremely long strings with the result that the sections of dragging chains create actual furrows on the seabed). An alternative technique consists in keeping the pipe in a semi-submersed position, i.e. directly resting on the seabed but with a lightened weight. In this case, controlled buoyancy floats can be used: for example, U.S. Pat. No. 4,121,529 describes an inflatable float capable of maintaining a constant buoyancy according to the depth, i.e. with the possibility of varying it as required again with reference to the depth. A combination of fixed and mobile buoyancy inflatable floats can be used.